Deploying optical network elements (ONEs) to form an optical network is a difficult and expensive proposition: network providers need to correctly anticipate customer demand while building reliable networks as inexpensively as possible. In addition, network providers must also anticipate future technological developments, such as increased data rates, to simplify network upgrades. In part, network providers attempt to minimize cost and reduce network complexity by deploying ONEs, such as optical amplifiers, in a way that minimizes the required power while ensuring sufficient signal fidelity.
In digital communication schemes, such as those employed in optical networks, signal fidelity may be characterized by a bit error rate (BER). Simply put, the BER is defined by how frequently a receiver detects a bit incorrectly, that is, how often the receiver mistakes a representation of a logical ‘1’ for a representation of a logical ‘0’ or vice versa. Lower BERs are better; ideal (i.e., noise-free) receivers operate with BERs of zero (0), but shot noise and thermal noise at real receivers cause bit detection errors, raising BERs to measurable levels.
Currently, the target BER for optical networks is on the order of 10−12. To meet the target BER, network providers must guarantee a minimum optical signal-to-noise ratio (OSNR) at the receiver. The OSNR is usually defined as the ratio of the optical signal power Ps to the optical noise power Pn in a given channel bandwidth,
                    OSNR        =                  10          ·                                                    log                10                            ⁡                              (                                                      P                    s                                                        P                    n                                                  )                                      .                                              Equation        ⁢                                  ⁢        1            For digital signals, the detected power switches between a high level and a low level at a given data, or bit rate. In optical networks, the high and low levels can be defined in terms of a number of photons: for example, a 5 mW, 40 GHz optical signal in the Wavelength Division Multiplexing (WDM) C band may have a corresponding high level of about 106 photons and a low level of 0 photons. In a shot-noise limited receiver, a signal of 106 photons has an OSNR of 30 dB.
Because bits can be defined in terms of photons, the bit rate can be defined in terms of photons per second. As the bit rate increases, the number of photons per bit decreases given a constant optical power (i.e., spreading a constant number of photons per second over a larger number of bits per second reduces the photons per bit). The increased bit rate also leads to a decreased OSNR—the bandwidth increases, but the signal power remains constant, whereas the receiver noise power increases given a relatively constant noise power spectral density. Eventually, increasing the bit rate depresses the OSNR too far, pushing the BER above acceptable levels. In optical networks that use direct detection, the BER is related to the OSNR according to the relation
                              BER          ⁢                                          ⁢          •          ⁢                                    1              2                        ·                                          log                10                            ⁡                              (                OSNR                )                                                    ,                            Relation        ⁢                                  ⁢        2            where the OSNR is in linear units. As shown in Relation 2, maintaining a minimum BER while increasing the bit rate requires increasing the OSNR.
As light propagates through a network, however, it is absorbed and scattered, reducing the signal power and the OSNR. In addition, signals propagating through optical fiber suffer from loss due to four-wave mixing, chromatic dispersion, and polarization mode dispersion, further reducing the OSNR. In long-haul and metro optical networks, optical amplification boosts the signal power, but may also add amplified spontaneous emission (ASE) noise, potentially offsetting any increase in OSNR. Because ASE noise spreads over a very broad bandwidth and OSNR depends on the noise power in a given bandwidth, ASE noise does not directly contribute to the OSNR. Instead, the ASE noise beats against the signal at the photodetector, producing a noise current near the frequency of the detected signal.
At present, resetting OSNRs to acceptable levels in metropolitan WDM networks involves converting optical signals to electrical signals, processing the electrical signals, and converting the processed electrical signals into optical signals. This process, known as optical regeneration, or simply regeneration, boosts the OSNR but requires relatively expensive transponders. Unfortunately, the transponder complexity and cost increase with the data rate and the number of channels, making regeneration an unattractive option for maintaining OSNR in metro WDM networks.